This invention relates to a process for recovery of chromium. More particularly, it relates to a process for recovery of chromium from material containing cobalt, molybdenum and tungsten, especially waste material resulting from processing of cobalt.
Chromium is a strategic metal in the United States because of the nearly 100% import dependence and wide variety of important uses. Among the most critical uses for chromium is providing high temperature and oxidation resistance in both cobalt and nickel based superalloys. In order to be utilized in this application, the chromium must be of high purity. Other common uses of the metal, for example in stainless steel, have less stringent purity requirements.
The commercial ore of chromium is chromite which has the ideal composition of FeO.Cr.sub.2 O.sub.3. Actually, free substitution of divalent magnesium and trivalent iron and aluminum frequently occur.
Two strategies are commonly employed to isolate and purify chromium from chromite.
The first involves smelting the chromite with coke to produce a high-carbon ferrochrome alloy and a slag phase containing the impurities other than iron. The high carbon ferrochrome is then leached with hot sulfuric acid to dissolve the iron and chromium. Following addition of ammonium sulfate, the solution is cooled to precipitate ferrous ammonium sulfate. Chromium ammonium sulfate crystallizes after conditioning the solution for an extended period of time. Typically, the chromium ammonium sulfate is recrystallized to improve purity and then redissolved and relatively pure chromium metal is electro-won from solution.
The second method used to produce chromium involves oxidative roasting of chromite ore with sodium hydroxide or sodium carbonate. Water soluble sodium chromate is extracted together with any excess sodium salts. The extract is acidified with sulfuric acid and the excess sodium is removed by crystallization of sodium sulfate. Further acidification and concentration of the solution gives chromic acid as the product. The chromic acid can be reduced to the trivalent oxidation state, precipitated as chromic hydroxide, and calcined to Cr.sub.2 O.sub.3. Aluminothermic reduction of Cr.sub.2 O.sub.3 gives chromium metal.
In the processing of secondary metal sources such as scraps and sludges to recover cobalt, hydroxidic precipates containing chromium as well as cobalt, iron, nickel, molybdenum, tungsten, and other metals is obtained. The chromium content of the material typically ranges from less than 10 to 25% compared with 48% chromium in chromite ore. The other impurity levels also vary considerably from batch to batch, depending on the starting materials. Because of the chemical differences between chromite ore and the hydroxidic precipitates of process residues, particularly with regard to amounts and types of impurities, the methods developed to convert chromite ore to metal do not give satisfactory results when applied to process residues. The oxidative roasting process succeeds in separating the chromium from the first row transition metals but it remains contaminated with tungsten and molybdenum.
Therefore, a process for separating and recovering chromium from process residues, particularly those containing tungsten and molybdenum would be highly desirable and an advancement in the art.